1. Field of the Invention
The present invention relates to sheet stacking, more specifically control for stacking sheets that are discharged from an image forming apparatus on a plurality of sheet stacking units.
2. Description of the Related Art
In recent years, an image forming apparatus that forms an image on a sheet could discharge a large number of sheets at high speed. Consequently, there is a demand that a sheet stacking apparatus receive and stack sheets discharged from the image forming apparatus main body that is capable of stacking a large number of sheets while maintaining the stacking alignment of the sheets. Japanese Patent Application Laid-Open No. 2006-124052 discusses a sheet stacking apparatus (hereinafter referred to as a “stacker apparatus”) which responds to such a requirement.
FIG. 24 illustrates a cross-sectional view of a conventional stacker apparatus.
In a stacker apparatus 500, an inlet roller 501 receives a sheet which is discharged from an image forming apparatus main body. A conveyance roller pair 502 then delivers the sheet to a gripper 503. The gripper 503 grips and conveys the sheet so that a leading edge of the sheet abuts on a leading edge stopper 504. When the sheet abuts on the leading edge stopper 504, the gripper 503 releases the sheet to fall onto a sheet stacking tray 505. At this time, the sheet falls between the leading edge stopper 504 and a trailing edge stopper 508, so that the leading edge and the trailing edge of the sheet are aligned. Further, a side edge of the sheet which is perpendicular to a sheet conveyance direction is aligned by a width alignment mechanism (not illustrated) as necessary.
In the above-described conventional stacker apparatus, if a number of sheets that are stacked on the sheet stacking tray 505 reaches the maximum stacking capacity, or a print job ends before reaching the maximum stacking capacity, the sheets that are stacked on the sheet stacking tray 505 become ready for taking out.
However, in the above-described conventional stacker apparatus, when a number of sheets to be printed in a job is greater than or equal to the maximum stacking capacity, the sheets cannot be taken out until the sheet stacking tray 505 is fully stacked thereon. Further, when the fully-stacked sheet stacking tray 505 is transported to a separate bookbinding apparatus for bookbinding processing, if a number of prints which the bookbinding apparatus can process for one time is less than the maximum stacking capacity of the stacker apparatus, the start of the bookbinding process is delayed by the difference in the number of sheets handled by the two apparatuses. As a result, the bookbinding apparatus cannot be operated efficiently.
Further, if the sheets are strongly curled, the stacked sheets become more prone to collapse as the sheet stacking amount increases.
In order to prevent the sheets from collapsing, a job which prints a large number of sheets can be divided into a plurality of jobs that print fewer sheets. However, it is burdensome for a user to prepare a divided job, and productivity is degraded.